The preparation of polyvinyl chloride which is suitable for paste production, in accordance with either the continuous or discontinuous method of preparation, is known.
Use of the continuous method results in forming a paste PVC for plastisols which has a desired low viscosity in the high shear range, but a high viscosity at low shear rates. Also, the rise in paste viscosity per unit time is relatively high. Furthermore, these PVC compositions contain relatively high emulsifier concentrations. A high emulsifier concentration has undesirable effects on both the final PVC product and also on the preparation of the product.
The PVC compositions prepared in accordance with the discontinuous method can be polymerized by emulsion polymerization with markedly lower quantities of emulsifier, especially if the emulsifier is added according to the teachings of German Patent No. 1,964,029 or in accordance with the laid-open disclosure of Belgian Patent No. 656,985. In all instances, though, the plastisols which are produced in the aforementioned discontinuous methods exhibit a higher paste viscosity than those prepared by continuous polymerization.
It is also known to conduct vinyl chloride polymerization according to the so-called microsuspension method, as described, for example, in DAS No. 1,069,387. Even though processing by the spread coating method can be more difficult, due to the frequently occurring dilatancy of the pastes prepared with these types of PVC at higher shear gradients, still, the use of this paste polyvinyl chloride type is widespread. For stabilizing the microsuspension polymerization of the vinyl chloride monomers the use of long-chain alcohols has been disclosed, in addition to emulsifiers, for example in DOS No. 1,520,133. Other patent disclosures claim linear-chain aliphatic alcohols having a C atom range of 10 to 30. In many applications the lower homologs, such as C.sub.10 -C.sub.14, are preferred since their melting point lies. only slightly above room temperature and their handling as liquids requires only low energy expenditures. In contrast, the higher alcohols, starting with about C.sub.14 with their melting points above 50.degree. C., necessitate higher expenditures in energy and technology. However, the volatility of the lower homologs can be harmful in the finished plasticized polyvinyl chloride article. This volatility leads to "fogging", a name that has been coined by persons skilled in the art. Fogging is particularly undesirable when using PVC in automobile production since it can lead to a greasy, troublesome coating, for example, on the glass panes.
Organic compounds of low water but high VC solubility have been utilized as auxiliary dispersants in other discontinuous processes for vinyl chloride polymerization. Such embodiments are disclosed in:
DOS No. 2,850,105; DOS No. 2,742,178; EP No. 0,030,524; DOS No. 3,210,891; DOS No. 3,242,088 and German Patent Application No. P 33 43 766.1.
The polymerizations of the aforementioned cited six processes are performed with fatty alcohols as the auxiliary polymerizing component. The polymerization autoclave is charged with water, emulsifier, alcohol, optionally an initiator, and also additional conventional additives, and the mixture is heated under agitation to above the alcohol melting point. Before adding the vinyl chloride, the mixture is cooled, if required, to the polymerization temperature. Utilizing during this process a fatty alcohol which has a melting point markedly, e.g., about 10.degree. C. above, the polymerization temperature, i.e., e.g. about 40.degree. to 50.degree. C., a paste polyvinyl chloride is obtained which is suitable as a high viscosity paste, as demonstrated by comparative Test A in the specification, infra. It would be desirable to employ long-chain fatty alcohols, e.g., C.sub.20 up to C.sub.30, on account of their lower volatility. However, the melting points of these alcohols lie significantly above the polymerization temperature ordinarily set for producing paste polyvinyl chloride. This means, for example, that in a paste PVC which has a K-value of 70, fatty alcohols up to about C.sub.18 can be utilized. It is also possible to use alcohol mixtures of high-molecular and low-molecular fatty alcohols in order to lower the melting point. When raising the K-value of the polymer, the C chain length of at least part of the fatty alcohol component must be correspondingly shortened. These fatty alcohols, though, promote the undesirable fogging which is more pronounced, particularly at higher K-values. By using fatty alcohols, either alone or in a mixture, which have a melting point close to room temperature, a paste PVC is obtained suitable as a high-viscosity paste, as demonstrated by Comparative Test B, infra. This finding is pointed out in DOS No. 2,850,105, and it is also noted therein that the melting point of the auxiliary agent must not be substantially (10.degree. C.) below the polymerization temperature in order to avoid high-viscosity pastes and low stability of the dispersions, especially for dispersions with solid content of 40-60%.